라거맥주 효모 침전물의 유변학 및 형태학적 변화

Yeast sediment is used for pitching subsequent batches or for making yeast extract in lager beer fermentation. The rheological properties of dense yeast sediments and those stored at different temperatures and periods were investigated using a rotational viscometer. A dense suspension of yeast cells harvested at the stationary phase showed slight dilatancy (n = 1.19-1.25). The dilatant yeast suspension became more viscous after several days at room temperature as well as at 4°C. A high shear rate (170.7 s^-1) reduced shear thickening in the yeast suspension stored at room temperature for 2 and 4 d. However, dilatancy was restored and even increased in the yeast cells at 6 d. Flow cytometry showed that haploid, diploid, triploid, and tetraploid yeast cells are present in the stationary phase yeast cells, in which polyploid populations varied dynamically. Image analysis of SEM micrographs revealed that the yeast cells have a bimodal distribution in size and the distribution varied significantly in yeast sediment stored at room temperature. Thus, the results suggest that the rheological properties of yeast sediments related to morphological changes could be associated with polyploid populations. Accordingly, rotational viscometry can be applied to monitor the physiological status of yeast sediments in the stationary phase.